The man allegedly entered the service station and threatened the staff member with a large knife before running away with a sum of cash.

Police said about 10 minutes before the alleged robbery, a man, believed to be the offender, was seen acting suspiciously around the McDonalds and Woolworths Petrol station at the Lanyon Marketplace in Conder.

The man is described as Caucasian, in his 20s, and skinny. He was wearing a blue jumper with a grey hood, black tracksuit pants and was carrying a black bumbag.

Police said they believe he fled on foot to a waiting car, parked in Roope Close, Calwell.